1. Field of the Invention
The present invention relates to an infrared solid-state image sensor.
2. Related Art
Infrared rays have a feature that the permeability to smoke and fog is high as compared with visible rays. Therefore, infrared image sensing is possible day and night. Since temperature information of the subject can also be obtained, the infrared image sensing has a wide application range such as the defense field, surveillance camera, and fire detecting camera.
In recent years, development of “uncooled infrared solid-state image sensor” which does not need a cooling mechanism has become vigorous. The uncooled i.e., thermal infrared solid-state image sensor apparatus converts incident infrared rays having a wavelength of approximately 10 μm to heat by using an absorption structure, and converts a temperature change in a heat sensitive part caused by feeble heat to an electric signal by using some thermoelectric conversion element. The uncooled infrared solid-state image sensor apparatus obtains infrared image information by reading out the electric signal.
For example, an infrared sensor (infrared solid-state image sensor) using a silicon pn junction which converts a temperature change to a voltage change by supplying a determinate forward current is known (JP-A 2002-300475 (KOKAI)). This infrared sensor has a feature that mass production using a silicon LSI manufacturing process is made possible by using a SOI (Silicon on Insulator) substrate as a semiconductor substrate. Furthermore, the infrared sensor has a feature that the pixel structure can be formed with extreme simplicity because the row selection function is implemented by utilizing the rectifying characteristics of a silicon pn junction (diode) serving as the thermoelectric conversion element.
One of indexes representing the performance of the infrared sensor is NETD (Noise Equivalent Temperature Difference) which represents the temperature resolution of the infrared sensor. It is important to make the NETD small, i.e., make the detected temperature difference corresponding to the noise. For that purpose, it is necessary to raise the signal sensitivity and reduce the noise.
Threshold voltage clamp processing for reducing the influence of threshold dispersion of an amplifying transistor is described in JP-A 2002-300475 (KOKAI). If a sampling transistor turns on, then in the threshold voltage clamp processing, negative charge is stored in the gate of an amplifying transistor capacitive-coupled to a signal line. At this time, it is preferable to converge the voltage across coupling capacitance between the signal line and the amplifying transistor to (Vdd−Vref)−Vth. Here, Vdd is a bias voltage supplied to a pixel by a row selection circuit, Vref is a voltage supplied from a constant load transistor to the signal line, and Vth is a threshold voltage of the pixel. In the threshold voltage clamp processing, dispersion of the threshold voltage among amplifying transistors in each column can be compensated. In each column, however, the noise component existing on the signal line is held the moment threshold voltage clamp is conducted, and thereafter the information is always referenced at the time of row selection. This results in a problem that longitudinal streak noise appears.